Hold codex in thinking while a turn is running

Codex uses the osc_title_stability idle strategy, but it draws its
progress in the pane body ('Working … esc to interrupt'), not the OSC
title. The title goes stable mid-turn, so ~2s later the classifier
declared codex idle while it was still working. Add a thinking-promoter
pattern ((?i)esc to interrupt) to the codex built-in preset; classify()
checks promoter regexes against the rendered screen before the
title-stability verdict, so codex stays in thinking until the turn's
in-progress footer actually disappears.

Resolves the [CODEX IDLE] TODO item.

internal/app/idle_test.go

@@ -57,6 +57,21 @@ func TestClassifyTitleStability(t *testing.T) {
 	}
 }
 
+func TestClassifyTitleStabilityThinkingPatternOverridesIdle(t *testing.T) {
+	cfg := &resolvedIdleDetection{
+		strategy:        StrategyOSCTitleStability,
+		idleThresholdMS: 2000,
+		thinkingRegexes: []*regexp.Regexp{mustCompile(t, `(?i)esc to interrupt`)},
+	}
+	screen := []byte("• Working (5s • esc to interrupt)")
+	if got, _ := classify(cfg, false, false, 9999, 5000, "codex", nil, screen); got != StateThinking {
+		t.Fatalf("thinking screen marker: got %q want %q", got, StateThinking)
+	}
+	if got, _ := classify(cfg, false, false, 9999, 5000, "codex", nil, []byte(">_")); got != StateIdle {
+		t.Fatalf("stable title without marker: got %q want %q", got, StateIdle)
+	}
+}
+
 func TestClassifyTitleStatus(t *testing.T) {
 	cfg := &resolvedIdleDetection{
 		strategy:        StrategyOSCTitleStatus,